Methods and compositions for highly purified boron nitride nanotubes

ABSTRACT

Herein we describe purified boron nitride nanotube compositions substantially free from hexagonal boron nitride. The compositions have a mass ratio of boron nitride nanotubes to hexagonal boron nitride of at least 100. Methods are provided for producing said purified boron nitride nanotube compositions wherein impure compositions are subjected to heating with a C5 to C11 hydrocarbon solvent under specified conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Patent Application No. 62/793,621, filed Jan. 17, 2019, theentire disclosure of which is incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION Field of the Invention

The field of the invention relates to methods of preparing purifiedboron nitride nanotubes (BNNT or BNNTs) and purified compositions ofBNNT.

Description of the Related Art

Due to their unique mechanical and thermal properties, interest in BNNThas grown dramatically over the past two decades. Like carbon nanotubes,BNNTs display exceptional strength. Despite being electricallyinsulating semiconductors, BNNTs possess high thermal conductivity. Theyare promising materials for many applications.

Realization of these promising applications has been hindered bysignificant challenges in BNNT synthesis and purification which hasproven far more difficult, for example, than carbon nanotubes (CNTs).For example, in WO20181024231 A1, Dushatinski describes problems withprior art synthesis and purification approaches for BNNT. Depending onthe synthesis conditions, as-synthesized BNNTs have substantial amountsof small boron-containing particles in the form of boron, amorphousboron nitride (a-BN), and hexagonal boron nitride (h-BN). Depending onthe synthesis conditions, these small boron-containing particles canaccount for 5% to 95% of the mass of the as-synthesized materials.

These impurities are problematic, as, for example, they can reduce theBNNT surface area, reduce strength, and/or reduce thermal conductivityof the BNNT materials. These impurities can further compromise theinterface of BNNTs with other materials and thus diminish their abilityto be dispersed, and/or to transfer mechanical load and heat across suchinterfaces in nanocomposites, with the consequence of reduced structuraland thermal performance of the BNNT composites.

Accordingly, methods have been sought for purification of as-synthesizedBNNTs. Methods for removing boron and boron oxides are known in theprior art and have been reasonably effective. In contrast, prior artmethods have been deficient in removing h-BN, which is challenging toremove from impure BNNTs (due to its chemical similarity with BNNTs),without damaging the BNNTs. These prior art methods can be costly,time-intensive, can reduce yields of BNNTs, and can damage BNNTs.

There is a need in the art for a method that virtually completelyremoves h-BN impurities without damaging the BNNTs, and there is a needin the art for high-purity BNNT compositions.

BRIEF SUMMARY OF THE INVENTION

The present disclosure relates to novel compositions of boron nitridenanotubes, and methods to produce said compositions. More specifically,the present disclosure relates to a method for the removal of boronnitride impurities and h-BN sheets from BNNT compositions using alow-temperature, non-destructive, hydrocarbon solvent-based method forthe purification of impure BNNTs. The present disclosure relates topurified boron nitride nanotubes having a mass ratio of boron nitridenanotube to hexagonal boron nitride exceeding 100. A process is providedfor producing said purified BNNT compositions comprising: (a) mixing animpure BNNT sample with a hydrocarbon solvent; (b) heating said mixtureof said impure BNNT sample and said hydrocarbon solvent for a period oftime; and (c) separating said hydrocarbon solvent from said BNNT sample;wherein said step of heating is performed at a temperature within 50° C.of the boiling point of said hydrocarbon solvent, wherein said heatingis performed for a period exceeding five minutes, and wherein saidhydrocarbon solvent comprises one or more hydrocarbons possessing atleast 5 carbon atoms and no more than 11 carbon atoms.

Suitable hydrocarbon solvents include C₅ to C₁₁ alkanes, C₅ to C₁₁cycloalkanes, C₅ to C₁₁ alkenes, C₅ to C₁₁ cycloalkenes, C₅ to C₁₁isoalkanes, and mixtures thereof. For example, suitable C₅ to C₁₁ alkanehydrocarbon solvents include pentane, hexane, heptane, octane, nonane,and decane. In some embodiments, heptane is the hydrocarbon solvent.While other compounds can be included in the C₅ to C₁₁ hydrocarbonsolvent, whether intentionally or unintentionally, the concentration ofC₅ to C₁₁ hydrocarbons in the C₅ to C₁₁ hydrocarbon solvent must be atleast 90% by weight.

In some preferred embodiments, during the step of heating thehydrocarbon solvent with the BNNT material, the heating temperature isthe boiling point of the hydrocarbon solvent. In other embodiments, themixture must be heated to at least 70° C. In some embodiments, theheating temperature is within 50° C. of the boiling point of thehydrocarbon solvent, or within 25° C., or within 10° C. In all suitableembodiments, the temperature is below 300° C., and preferably below 200°C. In some embodiments, the temperature is below 175° C., or below 150°C., or below 125° C., or below 100° C.

In some embodiments, the step of heating is performed in a pressurevessel. In other embodiments, the purification is performed in a Soxhletapparatus, or a suitable flask with a condenser, or simply by exposingthe BNNT to the hydrocarbon vapor in any suitable container. Use ofagitation, including sonication, can be used to enhance the process.

In some embodiments, the BNNT can be separated from the hydrocarbonsolvent simply by pouring off, decanting, or filtering the solvent.

The purified BNNT compositions described herein are substantially freeof h-BN impurities. Such h-BN impurities are known in the art to be verydifficult to remove from impure BNNT samples. More specifically, thepurified boron nitride nanotubes, as described herein, have a mass ratioof boron nitride nanotube to hexagonal boron nitride exceeding 100. Inother words, any boron atom in the purified BNNT is roughly 100 timesmore likely to be incorporated into a boron nitride nanotube than inhexagonal boron nitride. In some embodiments, the purified boron nitridenanotubes have a molar ratio of boron nitride nanotube to hexagonalboron nitride exceeding 200, or 300, or 400, or 500, or 1,000. Intypical embodiments, the purified boron nitride nanotubes have aBNNT/h-BN X-ray diffraction spectral ratio of at least 100.

The purified BNNT compositions described herein can be used, forexample, to enhance the mechanical and thermal conductivity of polymercomposites. BNNT is known through theoretical and experimental reportsto be 60 times stronger than steel. Thus, the addition of smallquantities of BNNT to a high performance polymer such as an epoxy orpolyimide has the potential to significantly increase the mechanicalperformance properties of the part. BNNT has six times the thermalconductivity of copper but, unlike most highly thermally conductivematerials, it is an electrical insulator. Removal of heat is the majorfactor limiting the development of smaller and more powerful electronicdevices. Addition of BNNT to polymers used to support high power diodeswould make it possible to conduct heat away from the diode. Thus,electronic components can potentially be made much more powerful andmuch smaller.

BRIEF DESCRIPTION OF THE DRAWINGS

The summary above, and the following detailed description, will bebetter understood in view of the drawings which depict details ofpreferred embodiments.

FIG. 1A shows a field emission scanning electron microscopy (FESEM)image of as-synthesized BNNT prior to purification. FIG. 1B shows anFESEM image of BNNT after purification according to the methodsdescribed herein. FIG. 1C shows an FESEM image of heptane purificationresidue removed from as-synthesized BNNT, and FIG. 1D shows a highermagnification FESEM image of heptane purification residue removed fromas-synthesized BNNT.

FIGS. 2A and 2B show overlaid X-ray diffraction (XRD) patterns ofas-produced BNNT and purified BNNT. FIG. 2A compares an XRD image ofas-produced BNNT with literature XRD values of h-BN, showing that theas-produced BNNT has a sharp shoulder peak around 26.69° which isconsistent with the literature peak of h-BN at 26.63°. FIG. 2B comparesXRD images of as-produced BNNT with purified BNNT, and demonstrates theabsence of the sharp shoulder peak at 26.69° in the purified BNNT.

FIG. 3A shows overlaid Raman spectra of purified BNNT and as-producedBNNT, demonstrating the lack of a pronounced peak around 1366 cm⁻¹ inthe purified BNNT. FIG. 3B shows overlaid Raman spectra of h-BN powderand residue material removed from as-produced BNNT during thepurification process, both of which have a strong peak in the 1366 cm⁻¹region.

FIG. 4A shows an FESEM image of a BNNT sample prior to a highertemperature heptane purification procedure. FIG. 4B shows an FESEM imageof the BNNT sample after the higher temperature heptane procedure whichdamaged the BNNTs.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure is directed to purified boron nitride nanotubecompositions and methods for producing such compositions.

The term “boron nitride nanotube(s)”, abbreviated BNNT or alternativelyBNNTs, refers to cylindrical boron nitride structures withsub-micrometer diameters and lengths exceeding 1 micrometer. They are apolymorph of boron nitride, and have a theoretical boron to nitrogenatomic ratio of one. Boron nitride nanotube compositions are producedwith significant levels of impurities, including boron, amorphous boronnitride, and hexagonal boron nitride. The term “hexagonal boronnitride”, abbreviated as h-BN, includes both h-BN nanocages and h-BNnanosheets.

The term “as-produced boron nitride nanotubes”, also referred to as“as-synthesized boron nitride nanotubes” and abbreviated as “as-producedBNNT”, means compositions of BNNT that have been produced but have notbeen substantially freed from all impurities.

The term “purified boron nitride nanotube compositions”, abbreviated as“purified BNNT”, means BNNT compositions purified such that any boronatom in the purified BNNT composition is 100 times more likely to beincorporated into a boron nitride nanotube than into hexagonal boronnitride.

The purification ratio can be determined by analyzing X-ray diffractionspectra. More specifically, the BNNT/h-BN X-ray diffraction spectralpeak ratio refers to the ratio of the peak height (a.u.) at the mainpeak of the XRD spectrum for BNNT (around 25.4°) to the peak height(a.u.) at the shoulder peak around 26.69°.

Any BNNT compositions can be used as starting materials. Such BNNTs canbe produced using any methods suitable for producing BNNTs, includingbut not limited to high-temperature/pressure (HTP) methods,hydrogen-assisted BNNT synthesis (HABS), extended-pressureinductively-coupled plasma (EPIC), and ball milling/annealing.

The present disclosure relates to purified boron nitride nanotubeshaving a mass ratio of boron nitride nanotube to hexagonal boron nitrideexceeding 100. A process is provided for producing said purified BNNTcompositions comprising: (a) mixing an impure BNNT sample with ahydrocarbon solvent; (b) heating said mixture of said impure BNNT sampleand said hydrocarbon solvent for a period of time; and (c) separatingsaid hydrocarbon solvent from said BNNT sample; wherein said step ofheating is performed at a temperature within 50° C. of the boiling pointof said hydrocarbon solvent, wherein said heating is performed for aperiod exceeding five minutes, and wherein said hydrocarbon solventcomprises one or more hydrocarbons possessing at least 5 carbon atomsand no more than 11 carbon atoms.

In some embodiments, said step of heating is performed in a pressurevessel that can permit pressures many times atmospheric pressure.Alternatively, the purification can be performed in a Soxhlet apparatus,or a suitable flask with a condenser, or simply by exposing the BNNT tothe hydrocarbon vapor in any suitable container.

Suitable hydrocarbon solvents include C₅ to C₁₁ alkanes, C₅ to C₁₁cycloalkanes, C₅ to C₁₁ alkenes, C₅ to C₁₁ cycloalkenes, C₅ to C₁₁isoalkanes, and mixtures thereof. For example, suitable C₅ to C₁₁ alkanehydrocarbon solvents include pentane, hexane, heptane, octane, nonane,and decane. In some embodiments, heptane is the hydrocarbon solvent.While other compounds can be included in the C₅ to C₁₁ hydrocarbonsolvent, whether intentionally or unintentionally, the concentration ofC₅ to C₁₁ hydrocarbons in the C₅ to C₁₁ hydrocarbon solvent must be atleast 90% by weight.

For example, pentane is an effective solvent for BNNT purification whenused according to the methods described herein. Hydrocarbons shorterthan pentane are gases at room temperature.

The C₁₂ hydrocarbon dodecane was ineffective when used in an attempt topurify BNNTs. Raman spectroscopy showed that h-BN was not removed from aBNNT sample treated with dodecane according to the methods describedherein. More polar solvents such as isopropanol were also found to beineffective.

Without wishing to be bound by theory, it is believed that the vaporpressure of the heated solvent is suitable to liberate the impuritiesfrom the BNNT structure, leaving purified BNNT. This process isparticularly effective for purifying away h-BN. For example, hydrocarbonmolecules can diffuse into cleavage spaces between planar h-BN andcurved BNNT, and the trapped hydrocarbon can exert pressure to driveapart the h-BN and the BNNT. Alternatively, hydrocarbon molecules candiffuse into the interior of a BNNT, and increased pressure can inducesmall changes in the tube conformation, potentially releasing impuritiesin the process.

Accordingly, it is important to have significant vapor pressure. In someembodiments, the solvent is heated such that the vapor pressure of thesolvent exceeds 300 mm Hg, or exceeds 400 mm Hg. In some preferredembodiments, during the step of heating the hydrocarbon solvent with theBNNT material, the heating temperature is the boiling point of thehydrocarbon solvent. In other embodiments, the mixture must be heated toat least 70° C. In some embodiments, the heating temperature is within50° C. of the boiling point of the hydrocarbon solvent, or within 25°C., or within 10° C.

If the pressure is too high, the BNNT can be destroyed, as is describedlater in Example 2 in the specification. Accordingly, there is a vaporpressure range for which the purification process is particularlyeffective. That vapor pressure range when the solvent is heptane isbetween about 300 mm Hg and 1,150 mm Hg, preferably between about 350 mmHg and 1,000 mm Hg, or between about 400 mm Hg and 900 mm Hg. Thepressure can be cycled such that BNNTs are subjected to fluctuations inpressure, which can facilitate sequential contraction and relaxation ofthe BNNTs.

The duration of heating at the requisite temperature is at least fiveminutes, and can be any longer duration that provides the desiredpurification without substantial damage to the BNNTs. For example, theheating can be performed for at least five minutes, or at least tenminutes, or at least 30 minutes. The heating can be performed in oneevent, or can be performed in a cyclic heating pattern comprisingalternate heating and cooling. Use of agitation such as sonicationduring heating can be used to enhance the process.

EXAMPLES

The examples that follow are intended in no way to limit the scope ofthis invention but are provided to illustrate representative embodimentsof the present invention. Many other embodiments of this invention willbe apparent to one skilled in the art.

Example 1

As-produced BNNT material was synthesized using a high temperaturepressure method coupled with a procedure to remove boron impurities. Theas-produced BNNTs were further purified as described below.

Removal of boron nitride and purification of BNNT was accomplished usingthe low temperature hydrocarbon procedure described herein. Thisprocedure involved mixing 50 mg of as-produced BNNT with 15 mL ofheptane (Sigma Aldrich, Raleigh, N.C., USA) in a 21 mL Ace pressure tube(Ace Glass Incorporation, Vineland, N.J., USA). The system was heatedand maintained at 90° C. for 5 hours in an oil bath. The system wascooled to room temperature before the pressure tube was opened. Heptanewas separated from the BNNT sample by decantation. The purified BNNT wasdried in a vacuum oven at −1 atm, 250° C. overnight. The decantedheptane solution containing the BN impurities was concentrated in thevacuum oven at 60° C. for further investigation.

Field emission scanning electron microscopy (FESEM) data was capturedusing Hitachi S-4700 field emission scanning electron microscope. Thesamples were then dispersed in isopropanol, pipetted onto graphite tape,spin dried, and sputter coated.

Raman data was collected using a Renishaw inVia dispersive Ramanspectrometer using 514 nm with excitation power 10-20 mW and a 100×objective with an N.A. of 0.65. The sample preparation for the residualimpurities was done by drop casting the concentrated heptane solution ona cleaned silicon wafer. The Raman spectrum had a broad background peakover the range 500-7000 cm⁻¹ that was due to fluorescence. Using MATLABsoftware, this background data was fitted using second order polynomialand subtracted. To get rid of noise, the data was subsequently smoothedusing 10-point averaging. The purified BNNT signal noise was calculatedusing root mean square method (RMS) to define the detection limit of thetechnique.

X-ray diffraction (XRD) analysis was performed using a Bruker SMART APEXII diffractometer equipped with an APEX II CCD Detector and a copper Kαsource (wavelength λ=1.54 Å). The samples were prepared by compressing20 mg of material into 3 mm discs. Due to the inhomogeneity of the BNNTsamples, different locations on the BNNT disc were measured. The XRDpattern signal noise was calculated using the root mean square method(RMS) which enabled us to define the detection limit of the technique tobe impressively low, between 0.2-0.4%.

Field emission scanning electron microscopy (FESEM) was conducted beforeand after the purification. As seen in FIG. 1A, the as-produced BNNTconsists of multiple nanoscale tube networks that entangle and extend tomicrons in length. This structure is generally understood to be resultof the BNNT growth process which has been hypothesized to originate fromdroplets of pure boron contained in fullerene-like BN cages(“nano-cocoons”). The larger impurities seen entangled within BNNTs maybe the remnants of these BN nano-cocoons and BN nanoparticles, or theycould be other impurities. As seen in FIG. 1A, these impurities (whichappear as bumps along the long tubes) form aggregates on the tubesurface and nodes. FIG. 1B shows the BNNTs after purification accordingto the methods described above. Visual impurities on the surface of theBNNTs are nearly absent, in stark contrast to FIG. 1A. Note that theBNNTs also appear to be thinner after the purification treatment.

Analyzing the decanted cleaning residue collected after drying theheptane filtrate yielded important insight into the composition of thematerial removed from the nanotubes via the purification method. FESEManalysis suggests that the impurities consisted of an abundance of smallparticles. FIG. 1C shows a representative FESEM image of a micron-sizedparticle hinting at a stacked surface structure. A higher-resolutionFESEM image of the material is shown in FIG. 1D.

To further understand the cleaning process, we carried out X-raydiffraction (XRD) of the BNNT material before and after cleaning, whichis shown in FIG. 2. The as-produced BNNT X-ray pattern shown in bothFIG. 2A and FIG. 2B has a pronounced series of peaks in the range2θ=20°-30°, corresponding to d-spacings in the range 4.43 Å-2.97 Å. Thisrange overlaps well with literature values for inter-layer spacingswithin multi-layer, 2D materials such as graphite or h-BN. The spectrumfeatures a second series in the range 2θ=40°-55°, corresponding tod-spacings in the range 2.25 Å-1.67 Å, likely representing theintra-layer spacings between the atoms of the in-plane honeycomblattice.

The first peak series was analyzed using Lorentzian peak fitting, withthe main peak at 25.33° and a sharp sub-peak on the right (26.69°) withFWHMs of 2.30° and 0.18°, respectively. To help interpret the measuredBNNT peak positions, we overlaid h-BN peaks based on literature valuesas shown in FIG. 2A. There is a remarkable agreement between theposition of the sharp 26.69° BNNT sub-peak on the right of theas-produced BNNT and the h-BN peak at 2θ=26.63°. The presence of thish-BN peak suggests that the as-produced BNNT contained a significantamount of h-BN.

When XRD was carried out on the purified BNNT compositions (purifiedusing heptane as a solvent in accordance with the methods of thedisclosure), we found that the main peak retained virtually the sameposition, shifting slightly to 25.46° (with a narrower FWHM 1.82°) asshown in FIG. 2B, which overlays the spectra of purified BNNT andas-produced BNNT. Importantly, the sharp sub-peak at 26.69° was absentin the purified BNNT spectrum, suggesting that the methods describedherein removed virtually all of the h-BN from the as-produced BNNT. Inthe 2θ=40°-55° range, the spectra of the as-produced and purifiedmaterials look very similar, indicating that the in-plane lattice of thepurified BNNT is not significantly altered by the purification proceduredescribed herein.

To assess the presence of remaining h-BN and/or quantify the degree ofh-BN removal, a Gaussian was fitted to the dominating main peak of thepurified sample and subtracted from the purified XRD spectrum in FIG. 2.This procedure allowed us to inspect all remaining features in thespectrum at greater amplification. Nevertheless, the XRD peak at 26.69°was not detectable above the general noise level of approximately 0.5intensity units (RMS noise). Having verified the absence of this h-BNpeak, more investigation was conducted to quantify the sensitivity ofthe XRD measurement and to define the detection limit of the XRDtechnique. First, the peak heights were determined using Gaussianfitting and are shown in Table 1. Then, the noise of the purified BNNTpattern was determined using the root mean square method (RMS). Theheight of the second Gaussian peak of the as-produced BNNT was comparedto the noise RMS. Using the ratio of the noise to the peak heightindicates that the h-BN detection limit is 0.002. The absence of thissub-peak around 26.69° in the XRD spectrum of the purified BNNT showsthat the purification method described herein succeeded in removing atleast 99.8 percent of the h-BN impurities.

TABLE 1 XRD fitting parameters Peak FWHM Height Sample ID Peak position[°] [°] [a.u.] R² As-produced Main 25.33 2.30 1000.0 0.94 BNNT h-BN26.69 0.18 284.1 Purified BNNT Main 25.42 1.82 1000.0 0.98

In this example, the as-produced BNNT had a BNNT/h-BN X-ray diffractionspectral peak ratio of 3.52 (i.e., the ratio of the heights of the mainpeak to the h-BN peak, corresponding to 1,000 divided by 284.1). Thepurified BNNT has a BNNT/h-BN X-ray diffraction spectral peak ratio ofat least 1,000 (i.e., the peak height of 1,000 a.u. at the main BNNTpeak of 25.42° divided by the h-BN peak height which was notdistinguishable above the general noise level of 0.5 a.u.).

We further analyzed the effectiveness of our purification technique toremove h-BN using non-resonant Raman spectroscopy with an excitationwavelength λ=514 nm (the expected bandgap of BNNTs is 5.5 eV,corresponding to a photon wavelength of 225 nm, while for h-BN thecorresponding wavelength is 215 nm). According to literature (Nemanich,R J et al., “Light scattering study of boron nitride microcrystals”.Physical Review B 23, 6348-6356 (1981)), h-BN is expected to feature apronounced peak at 1365 cm⁻¹, whereas BNNTs have been shown not tofeature this peak in non-resonant Raman spectroscopy.

FIG. 3A shows overlaid Raman spectra of (i) as-produced BNNT and (ii)purified BNNT. The spectrum of the as-produced BNNT has one pronouncedpeak, located at 1368.5 cm⁻¹, while the purified BNNT featured no peakin this spectral region. FIG. 3B shows overlaid spectra of (i) h-BN and(ii) the purification residue removed from as-produced BNNT whenpurified using the methods disclosed herein. Each spectrum features onepronounced peak, located at 1364.6 cm⁻¹ and 1367.4 cm⁻¹, respectively.Table 2 lists the corresponding peak positions and full widths at halfmaximum (FWHM) from the peak fitting parameters of the Raman peaks. Theobserved h-BN peak at 1364.6 cm⁻¹ is in excellent agreement with theliterature value.

TABLE 2 Raman FWHM and peak position based on peak fitting FWHM PeakSample [cm⁻¹] position [cm⁻¹] R² h-BN 12.5 1364.6 ± 0.1 0.99 Residuematerial 26 1367.4 ± 0.2 0.97 As-Produced BNNT 36 1368.5 ± 0.2 0.98

The Raman data for the as-produced BNNT (FIG. 3A) shows a symmetric peaksimilar to the peak in the h-BN and residue material. The BNNT has nopeak in the non-resonance Raman, suggesting that the as-produced BNNTcontains h-BN impurities, which is further support for the XRD findingsdescribed above. Furthermore, the peak position of the residue peak isvery close to the h-BN peak, which suggests that the collectedpurification residue contains the removed h-BN impurities.Interestingly, the peak position did not change much between theas-produced material (1368.5 cm⁻¹) and the residue (1367.4 cm⁻¹),suggesting that the size of the h-BN particles was not altered muchduring the purification procedure. Thus, we can conclude from our Ramananalysis that the as-produced BNNTs contain h-BN, and that ourpurification procedure removes substantially all of the h-BN, which isin full agreement with the XRD results.

Example 2

As-produced BNNT material was synthesized using a high temperaturepressure method coupled with a procedure to remove boron impurities. Theas-produced BNNTs were further purified as described below.

To test the range of the purification procedure described herein,as-produced BNNT was mixed with heptane (Sigma Aldrich, Raleigh, N.C.,USA) in an Ace pressure tube (Ace Glass Incorporation, Vineland, N.J.,USA). The system was heated and maintained at 120° C. for 5 hours in anoil bath, corresponding to a projected pressure of 1187 Torr (1.56 atm).The system was cooled to room temperature before the pressure tube wasopened. Heptane was separated from the BNNT sample by decantation. FIG.4A shows an FESEM image of the BNNT sample before the heptanepurification procedure, while FIG. 4B shows an FESEM image of the BNNTsample after the heptane procedure. FIG. 4B shows that the BNNTs shownin FIG. 4A were damaged by the relatively high pressure and temperatureheptane procedure used in this example.

Incorporation by Reference

All publications, patents, and patent applications cited herein arehereby expressly incorporated by reference in their entirety and for allpurposes to the same extent as if each was so individually denoted.

Equivalents

While specific embodiments of the subject invention have been discussed,the above specification is illustrative and not restrictive. Manyvariations of the invention will become apparent to those skilled in theart upon review of this specification. The full scope of the inventionshould be determined by reference to the claims, along with their fullscope of equivalents, and the specification, along with such variations.

The articles “a” and “an” are used herein to refer to one or to morethan one (i.e. to at least one) of the grammatical object of thearticle. By way of example, “a boron nitride nanotube” means one boronnitride nanotube or more than one boron nitride nanotube.

What is claimed is:
 1. Purified boron nitride nanotubes having a massratio of boron nitride nanotubes to hexagonal boron nitride exceeding100.
 2. The purified boron nitride nanotubes of claim 1, having aBNNT/h-BN X-ray diffraction spectral peak ratio of at least
 100. 3. Thepurified boron nitride nanotubes of claim 1, having a BNNT/h-BN X-raydiffraction spectral peak ratio of at least 100 and lacking asignificant Raman spectrum peak in the 1366 cm⁻¹ region.
 4. A method forproducing purified boron nitride nanotube compositions comprising thesteps of: mixing an impure BNNT sample with a hydrocarbon solvent;heating said mixture of said impure BNNT sample and said hydrocarbonsolvent for a period of time; and separating said hydrocarbon solventfrom said BNNT sample; wherein said step of heating is performed at atemperature within 50° C. of the boiling point of said hydrocarbonsolvent, wherein said heating is performed for a period exceeding fiveminutes, and wherein said hydrocarbon solvent comprises one or morehydrocarbons possessing at least five carbon atoms and no more thaneleven carbon atoms.
 5. The method of claim 4, wherein said hydrocarbonis heated such that its vapor pressure is at least 300 mm Hg, and lessthan 1,150 mm Hg.